203 
from the Lower Coal Measures of England . 
the hypoderm. In C. Felicis there are also thickened masses of hypoderm 
between the bundle strand and the partitions (Fig. io). On the upper 
surface these masses are surrounded by but slightly differentiated palisade, 
while on the lower they are surrounded by the lacunar tissue connected with 
the stomatiferous epidermis. 
The horizontal section shown in Fig. 3 is equidistant between the 
upper and lower surfaces of the leaf and thus escapes the hypodermal 
masses. The vascular bundles with their well-developed sheaths are 
approximately circular in section (Fig. 10), and relatively to the thickness 
of the leaf are larger than those of C. Wedekindi , Felix. In the parts 
of the leaf where division is most frequent, the bundle sheath is scarcely 
differentiated (Fig. 5). Correlated with the absence of the sheath the 
partition is more strongly constructed (Figs. 2 and 5), and the specimen 
approximates to the C. robustus type of Felix. 
When present the sheath is composed of layers of longitudinally 
disposed fibres. In some cases pits are shown on their walls, but as a rule 
the later deposited thickening layers of the wall are dissolved before 
petrifaction, and the pits are then no longer distinguishable. 
The innermost layer of the sheath is generally composed of cells 
of narrow calibre, and so the abaxial region of the bundle is partially com- 
posed of cells like those of the centrifugal primary xylem, which they 
slightly exceed in size. This sheath may be present even when no normal 
centrifugal elements may be differentiated (Fig. 8, Is). 
To turn to the vascular elements proper, we note that the upper part 
of the xylem consists of a few large, porose, centripetal tracheides which 
increase in number as the leaf thickens. A considerable number may 
be present in the basal part of the leaf, especially when the bundle is 
dividing (Fig. 6). In the non-dividing bundle the centripetal wood radiates 
off from a small protoxylem group, which in turn is often separated from the 
centrifugal elements by a layer of parenchyma. In transverse sections the 
centrifugal elements, which are small and spiro-reticulate, lie in an irregular 
crescent which reaches to the periphery of the centripetal wood above and 
the small-celled inner sheath laterally. Thus, if we regard the tracheides 
of the inner sheath as primary transfusion tissue derived from xylem, there 
seems more reason to connect it with the centrifugal xylem than with the 
centripetal. It will be observed from the text-figure that the tracheides of 
the inner sheath (Is) are of slightly wider calibre than those intervening 
between them and the protoxylem (Px). 
It is possibly the increasing development of this inner sheath in later 
forms that explains the more common absence of centrifugal wood in 
C. principalis as described by Dr. Marie C. Stopes. Below the centrifugal 
elements is a layer of xylem parenchyma which again is succeeded by the 
phloem (text-figure, P ). 
